The present invention relates to wireless communication systems and, more particularly, to a method for providing selectivity for remote or local reset as well as link status information propagation for nodes connected in a daisy chain via Common Public Radio Interface (CPRI).
CPRI is an industry standard aimed at defining a specification for the interface of base stations between Radio Equipment Controllers (REC) and Radio Equipment (RE). FIG. 1 shows a topology of a chain 100 including several nodes 102a-102e connected via the CPRI interface. Node 102a is a REC node, node 102e is a RE node, and nodes 102b-102d can be either a REC node or a RE node. As shown in node 102C, in order to support a CPRI daisy chain configuration, each node has a pair of CPRI units 104 connected through their auxiliary interfaces 106. The chain 100 can be located in one transmission node, e.g., an eNodeB or a relay node, or across more than one transmission node.
Link status information 108 like Loss of Signal (LOS), Loss of Frame (LOF) and other Layer 1 alarms such as Remote Alarm Indication (RAI) and Service Access Point (SAP) Defect Indication (SDI), are transmitted from one node to another in a point to point fashion. On the other hand, a Reset request 110 originated from the REC node 102a, or any intermediate node in the chain 100, which is the node 102b in this case, is propagated across the chain 100 in other than a point to point fashion. The local node 102c, on receiving the Reset request 110 from the previous adjacent node in the chain 100, should propagate the Reset request 110 to the next node 102d in the chain 100.
FIG. 2 is a schematic diagram illustrating the frame hierarchy and notation indices in the CPRI protocol. As illustrated in FIG. 2, a basic frame has 16 words, indexed with W=0 . . . 15; 256 time division multiplexed basic frames, indexed with X=0 . . . 255, form a hyperframe; and 150 hyperframes, indexed with Z=0 . . . 149, for a radio frame. The radio frame is a frame to be transmitted and received over a radio area (cell or sector) provided by a RE node or a REC node. The duration of a basic frame is about 260.4 ns.
As mentioned above, a basic frame has 16 words, W=0 . . . 15. The word at index W=0 is used for a control word, and the remaining words (W=1 . . . 15) are dedicated to the U-plane In-Phase and Quadrature (IQ) data transport (IQ data block). The length of the control word depends on the CPRI link rate. In the basic frame, each byte (8 bits) within a word is addressed with an index Y. Therefore, at the minimum CPRI link rate of 614.4 Mb/s, Y=0, which means that there are 128 bits in a basic frame, out of which 8 bits are for the control word and the remaining 120 bits are IQ data bits, while at the maximum CPRI link rate of 9830.4 Mb/s, Y=0 . . . 15, which means that there are a total of 2048 bits in a basic frame, out of which 128 bits are for the control word and the remaining 1920 bits are IQ data bits.
According to the CPRI protocol, the Reset request 110 and the link status information 108 are transmitted through bits 0 to 4 of the control word of the basic frame #Z.130 of each hyperframe. When an RE or an REC receives a valid Reset request 110, it should reset itself and forward the Reset request 110 by setting the lowest bit of the basic frame #Z.130 in downlink for at least 10 hyperframes, and acknowledge the reception of a valid Reset request 110 through a Reset ACK 112 by setting the lowest bit of the basic frame #Z.130 in uplink for at least 5 hyperframes on the same link. The link status information 108, such as RAI, SDI, LOS and LOF carried in bits 1 to 4 of the control word of the basic frame #Z.130, should be generated locally and transmitted to the next node in the chain.
FIG. 3 shows a conventional RE/REC node 300. In order to control the source of the 2048 bits of the basic frame, a set of 2048 software programmable mask bits is stored in a mask register 302 in the RE/REC node 300 and provided to a first CPRI unit 304a through the auxiliary interface 306a of the first CPRI unit 304a. If a mask bit is programmed to ‘0’, the data corresponding to it is generated locally and sent to the next CPRI unit in the chain 100. If the mask bit is programmed to ‘1’, the data bit received from the previous CPRI unit is forwarded to the next CPRI unit in the chain 100. There is only one set of 2048 mask bits for all 256 basic frames in a hyperframe. As illustrated in the node 102c (FIG. 1), like the mask register 302, there is another mask register (not shown) for storing a set of 2048 software programmable mask bits to be provided to a second CPRI unit 304b through a second auxiliary interface 306b of the second CPRI unit 304b. 
In order to provide selectivity for remote or local reset as well as link status information propagation for all nodes connected in a daisy chain via CPRI, software in the node 300 should be able to program the mask register 302 appropriately after transmission of the control word of the previous basic frame #Z.129 and before the transmission of the basic frame 2.130 begins. Since the duration of a basic frame is 260.4 ns, this poses a large overhead on the software, that is, to be able to change the mask value within 260.4 ns. Further, it is very difficult for the software to maintain sync with the data transmission occurring on the link, i.e., the software must determine precisely when the basic frame #Z.130 will be transmitted over a link so that it can appropriately program the mask register 302 at the start of the basic frame #Z.130. In addition, if the software of a particular RE/REC node is unresponsive, the reset request 110 may not be forwarded beyond that node. This may prevent the communication system from recovering from a system hung state, which may even require manually resetting the system nodes 102.
It is therefore desirable to reduce the software overhead in forwarding the Reset request to a next node and at the same time transmitting the link status information in point to point fashion, and to make the RE/REC node able to forward the reset request to the next node if the RE/REC node becomes unresponsive.